The present invention relates to the operation of a Fluidized Catalytic Cracking Unit (FCCU) comprising of the feed preheat unit, reactor, regenerator, wet gas compressor, the main fractionator and the downstream light ends processing towers. In particular, the present invention relates to determining when the process is deviating from normal operation and automatic generation of notifications isolating the abnormal portion of the process.
Catalytic cracking is one of the most important and widely used refinery processes for converting heavy oils into more valuable gasoline and lighter products. The process is carried out in the FCCU, which is the heart of the modern refinery. The FCCU is a complex and tightly integrated system comprising of the reactor and regenerator. FIG. 23 shows a typical FCCU layout. The fresh feed and recycle streams are preheated by heat exchangers and enter the unit at the base of the feed riser where they are mixed with the hot regenerated catalyst. The FCC process employs a catalyst in the form of very fine particles (˜70 microns) which behave as a fluid when aerated with a vapor. Average riser reactor temperatures are in the range of 900 to 1000 degF with oil feed temperatures from 500-800 degF and regenerator exit temperatures for catalyst from 1200 to 1500 F. The process involves contacting the hot oil feed with the catalyst in the feed riser line. The heat from the catalyst vaporizes the feed and brings it up to the desired reaction temperature. The cracking reactions start when the feed contacts the hot catalyst in the riser and continues until the oil vapors are separated from the catalyst in the reactor. As the cracking reaction progresses, the catalyst is progressively deactivated by the formation of coke in the surface of the catalyst. The spent catalyst flows into the regenerator and is reactivated by burning off the coke deposits with air. The flue gas and catalyst are separated in the cyclone precipitators. The fluidized catalyst is circulated continuously between the reaction zone and regeneration zone and acts as a vehicle to transfer heat from the regenerator to the oil feed and reactor. The catalyst and hydrocarbon vapors are separated mechanically and the oil remaining on the catalyst is removed by steam stripping before the catalyst enters the regenerator. The catalyst in some units is steam-stripped as it leaves the regenerator to remove adsorbed oxygen before the catalyst is contacted with the oil feed. The hydrocarbon vapors are sent to the synthetic crude fractionator for separation into liquid and gaseous products. These are then further refined in the downstream light ends towers to make gasoline and other saleable products. The complete schematic with FCCU and the downstream units is shown in FIG. 24.
Due to the complicated dynamic nature of the FCCU, abnormal process operations can easily result from various root causes that can escalate to serious problems and even cause plant shutdowns. These operations can have significant safety and economic implications ranging from lost production, equipment damage, environmental emissions, injuries and death. A primary job of the operator is to identify the cause of the abnormal situation and execute compensatory or corrective actions in a timely and efficient manner.
The current commercial practice is to use advanced process control applications to automatically adjust the process in response to minor process disturbances, to rely on human process intervention for moderate to severe abnormal operations, and to use automatic emergency process shutdown systems for very severe abnormal operations. The normal practice to notify the console operator of the start of an abnormal process operation is through process alarms. These alarms are triggered when key process measurements (temperatures, pressures, flows, levels and compositions) violate predefined static set of operating ranges. This notification technology is difficult to provide timely alarms while keeping low false positive rate when the key measurements are correlated for complicated processes such as FCCU.
There are more than 600 key process measurements, which cover the operation of a typical FCCU. Under the conventional Distributed Control System (DCS) system, the operator must survey this list of sensors and its trends, compare them with a mental knowledge of normal FCCU operation, and use his/her skill to discover the potential problems. Due to the very large number of sensors in an operating FCCU, abnormalities can be and are easily missed. With the current DCS based monitoring technology, the only automated detection assistance an operator has is the DCS alarm system which is based on the alarming of each sensor when it violates predetermined limits. In any large-scale complex process such as the FCCU, this type of notification is clearly a limitation as it often comes in too late for the operator to act on and mitigate the problem. The present invention provides a more effective notification to the operator of the FCCU.